The present invention relates to a CMOS (complementary metal oxide semiconductor) image sensor having enhanced photosensitivity and a method for fabricating the same.
As well known, to embody color images in an image sensor, color filters are arranged over photosensitive parts, which receive lights from the external and generate and accumulate photocharges. The color filter array (CFA) comprises three colors of red, green and blue, or those of yellow, magenta and cyan.
The image sensor includes the photosensitive parts sensing the lights and accumulating photocharges and logic circuit parts transforming the photocharges into electric signals and generating data. To improve the photosensitivity of the image sensor, there have been proceeded endeavors to increase the area ratio (fill factor) of the photosensitive parts in the unit pixel. However, there are fundamentally limits in such endeavors, because the logic circuit parts can not be completely eliminated and thus, the photosensitive parts exist in a limited area. Accordingly, in order to increase the photosensitivity, light-collecting technique has been appeared. Using the technique, the pathways of the incident lights injecting to the regions other than the photosensitive parts are changed and collected in the photosensitive parts. For the light collecting, the image sensor includes microlenses on the color filters.
FIG. 1 is a cross-sectional view schematically showing a CMOS image sensor fabricated in accordance with a prior art. Referring to the drawing, which depicts particularly in detail the structure deposited over a photodiode, the fabrication method of the image sensor according to the prior art will be described. A field insulating film 102 is formed on a silicon substrate 101 electrically to insulate elements and a photodiode 103 as a light-receiving element is formed. After that, interlayer-insulating film 104 is deposited and metal wire 105 is formed. Subsequently, in order to protect the element from moisture or scratch, an oxide film and a nitride film are, in turn, deposited to form element-protecting film 106, which is then etched to form pad open parts (not shown). The metal wire 105 is partly exposed through the pad open parts and will provide electrical contact of the element with the external. For accomplishing color image of the image sensor, color filter array 107 is then formed on the element-protecting film. A dyed photoresist may be normally used as a color filter material. After the formation of color filter array, microlens 108 is formed on the color filter array. The microlens 108 is to collect the incident light injected to the regions other than the photosensitive region.
In the processes, due to the step coverage of the protecting film 106, the thickness of the color filter array is larger in the upper position of the photodiode 103 than the other positions thereby decreasing transmittance of light penetrated through the color filter array. A large number of incident lights are refracted at the inclined or curved parts of the nitride film having high index of refraction (2.05) and proceeded into the regions other than the photodiode region, thus to decrease the photosensitivity.
Further, the poor flatness of the color filter array 107 also causes the figures of the formed microlenses 108 on the color filter array to be different from each other and thus the property uniformity of the microlenses to be deteriorated. Since the formed microlens 108 also has poor figures in the edges, the incident lights are irregularly refracted at the edges to decrease the number of condensed lights.
Meanwhile, as shown in the drawing, the sum of the thickness of the interlayer-insulating film 104, the protecting film 106, and the color filter 107 is not sufficiently enough to focus the incident lights onto the photodiode 103. That is to say, because the focal length of the microlens is longer than the total thickness, the light sensitivity becomes low.
It is, therefore, an object of the present invention to provide a method for fabricating a CMOS image sensor having enhanced reliability and light sensitivity to overcome the shortcoming of the prior art.
It is another object of the present invention to provide a CMOS image sensor having enhanced reliability and light sensitivity to overcome the shortcoming of the prior art.
By the present invention, the color filter corresponding to each photodiode can be uniform in the thickness. The residuals of the color filter materials on the wire-bonding pad can also be completely removed. The figures of the microlenses can still also be uniform. The total thickness of the films between the microlens and the photodiode can still also be fitted to the focal length of the microlens or established to focus the incident lights onto the photodiode.
In accordance with an aspect of the present invention, there is provided a method for fabricating a CMOS image sensor, which comprises the steps of: providing a substrate including photosensitive elements and metal wire; forming a first protecting film for protecting the elements over the substrate, covering the metal wire; forming a flattened spin-on-glass film on the first protecting film; forming a second protecting film for protecting the elements on the spin-on-glass film; and forming color filter patterns on the second protecting film.
The method further comprises the steps of forming a photoresist film for flattening on the color filter patterns and the second protecting film and forming microlenses on the photoresist film.
In accordance with another aspect of the present invention, there is provided a CMOS image sensor comprising: a substrate including a plurality of photosensitive elements; an interlayer-insulating film over the substrate; metal wire patterns on the interlayer-insulating film; a first protecting film on the interlayer-insulating film, covering the metal wire patterns; a flattened spin-on-glass film on the first protecting film; a second protecting film on the spin-on-glass film; color filter patterns on the second protecting film; a photoresist film for flattening on the color filter patterns and the second protecting film; and microlenses on the photoresist film.
Preferably, the formed spin-on-glass (SOG) film is etched back. The photoresist film is used as a mask for pad opening. That is to say, before the step of forming the microlenses, the photoresist film is selectively exposed at pad open regions and developed to be removed at the pad open regions. The first and second protecting films are then selectively etched to form the pads of metal wire at the regions. The photoresist plays a role of etching mask during the etching processes. The first protecting film is preferably an oxide film and the second protecting film is preferably an oxynitride film.
Preferably, the thickness of the photoresist is determined to focus the incident lights onto the photosensitive element. The thickness is preferably determined based on the focal length of the microlens in the well-known medium of, particularly, vacuum or air and the indices of refraction of the films between the microlens and the photosensitive element. It is preferable to form the photoresist with transparent material. The photosensitive element is preferably a photodiode and the image sensor may include some transistors as well as the photodiode.